The present invention concerns the non-destructive testing of materials, and more precisely the detection and characterization of defects in a heterogeneous material by ultrasound.
Ultrasounds are commonly used for implementing non-destructive testing of materials. To do this, an ultrasound transducer is used, placed on the surface of the material to be examined, which emits ultrasonic waves into the material. These waves propagate through the material and are reflected by it according to its structure. The transducer receives these waves, and analysis of them makes it possible to detect any defects in the material.
However, for a heterogeneous material, i.e. a polycrystalline material with a grain size in the order of the ultrasonic wavelength in this material, the phenomenon of diffusion of the ultrasonic wave by the material structure becomes predominant. This diffusion can then lead to the generation of a structure noise, i.e. an ultrasonic signal of non-negligible amplitude received by the transducer and exhibiting characteristics similar to those that a wave reflected by a defect would transmit, thus leading to a deterioration of the ability to detect the defects actually present in the material.
Indeed, insofar as the structure noise has temporal and spectral characteristics similar to those of the defect signatures forming the useful signal, the conventional approaches for processing ultrasound signals, by time or frequency filtering, deconvolution or wavelet-based projection turn out to be ineffective.
The patent application US 2007/0006651 A1 describes a method for non-destructive testing by means of ultrasonic waves, based on the comparison of the amplitude of the frequency spectrum of a selected part of the signal with a reference amplitude. This application mentions the possibility of taking the measurements at different positions and describes the combination of these measurements to obtain a measurement signal that is an average in the spatial sense. However, such a method is not completely satisfactory, and the signal remains noisy.